Electroforming

As in electroplating, the physical, chemical and mechanical properties of the electrodeposited metal must be controlled (in electroforming, the hardness. 

The fundamental principles of electroforming are identical to those of electroplating and, hence, much of the discussion in Chapter 8 is again relevant. Rather than seeking an even deposit, however, the objective may be to plate the metal in an uneven but controlled manner so as to produce the desired shape. 

The metal is deposited onto a mandrel which for some products (e.g. gauzes) will be a mixture of conducting and non-conducting surfaces. For the forming of many products the mandrel will also be of a shape and size determined by the process designer who may further make use of other aids when producing complex shapes. These might include  

Non-conducting shields in the anode mandrel gap to modify the local current density and, hence, the distribution of metal. 

Robber cathodes, i.e. cathodes which will not be part of the formed product and which act so as to reduce deposition at the part of the mandrel close to them. Their use will cause loss of both current and metal efficiency and, hence, should, whenever possible, be avoided. 

Several electrochemically based methods are employed in the metals processing industry because of their ability to manufacture metal articles or components which are difficult or impossible to produce by traditional mechanical workshop techniques. The most important methods are electroforming and a group used for the controlled removal of metal, i.e. electrochemical machining, grinding and deburring. 

Because the principles of electrochemistry are foreign to an industry based on mechanical engineering and the electrolytes necessary for an electrochemical process cause corrosion in the workshop, these methods have generally been developed by specialist companies or sections. This has minimized their impact on the engineering industry as a whole and their full potential has yet to be realized. 

It is clearly essential to separate the formed product from the mandrel without damage to the product and, if possible, to the mandrel since it can then be re-used. This is generally accomplished by using a cathode which is covered by a natural or chemically induced thick oxide layer. Suitable materials include titanium, chromium and steel. For some products, their shape predetermines that a permanent mandrel cannot be used. Non-permanent mandrels have to be constructed of a material which can be removed from the inside of the product and several techniques have been used. The non-permanent mandrel may be made from a low-melting-point metal (e.g. Zn, A1 or their alloys), a metal which may be removed by chemically etching or a non-metallic material (e.g. perspex, PVC or epoxy resins) soluble in organic solvents and plated by electrodeless deposition with a layer of silver or copper to make it conducting. 

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Nickel Sulfamate. Nickel sulfamate [13770-89-3] Ni(S02NH2)2 4H2O, commonly is used as an electrolyte ia nickel electroforming systems, where low stress deposits are required. As a crystalline entity for commercial purposes, nickel sulfamate never is isolated from its reaction mixture. It is prepared by the reaction of fine nickel powder or black nickel oxide with sulfamic acid ia hot water solution. Care must be exercised ia its preparation, and the reaction should be completed rapidly because sulfamic acid hydrolyzes readily to form sulfuric acid (57). 

Dissolution of Silver. Silver is dissolved by oxidising acids and alkaU metal cyanide solutions in the presence of oxygen. The latter method is the principal technique for dissolving silver from ore. Silver has extensive solubiUty in mercury (qv) and low melting metals such as sodium, potassium, and their mixtures. Cyanide solutions of silver are used for electroplating and electroforming. The silver is deposited at the cathode either as pure crystals or as layers on a mandrel. 

Nickel. Worldwide, nickel used in electroplating has averaged about 63,500 t annually from 1980—1990 (9). The United States uses about 18,000 t/yr, and Europe about the same quantity Japan consumes about 9,000 t, and another 9,000 t is used by the other Pacific rim countries. Canada and South America are reported to use about 4500 t aimuaHy. Electroforming apphcations consume another 4500 t of nickel worldwide. About half of this electroforming is done in the United States and Canada. Nickel deposited from autocatalytic solutions was estimated to account for 1600 t of nickel on a worldwide basis (10) in 1990. Nickel averaged 3.65/kg ia early 1993 (see Nickel and nickel alloys). 

Acid Copper. Bath compositions are shown in Table 8. The acid sulfate bath is by far the most widely used copper plating bath, both for plating and for electroforming and electrowinning. The fiuoborate baths have been Htde used in spite of the high current densities possible. Additional information can be found in the Hterature (98,99). 

Electroforrning is the production or reproduction of articles by electro deposition upon a mandrel or mold that is subsequendy separated from the deposit. The separated electro deposit becomes the manufactured article. Of all the metals, copper and nickel are most widely used in electroforming. Mandrels are of two types permanent or expendable. Permanent mandrels are treated in a variety of ways to passivate the surface so that the deposit has very Httie or no adhesion to the mandrel, and separation is easily accompHshed without damaging the mandrel. Expendable mandrels are used where the shape of the electroform would prohibit removal of the mandrel without damage. Low melting alloys, metals that can be chemically dissolved without attack on the electroform, plastics that can be dissolved in solvents, ate typical examples. 

Plating solutions used in nickel electroforming are primarily the Watts bath and the nickel sulfamate bath. Watts baths exhibit higher stress and require additives for stress control, which may affect other properties. Sulfamate baths produce much lower stress and are preferred where purer nickel or nickel—cobalt deposits ate needed. ASTM specifications are available that describe the mandrels and plating solutions (116,162). 

W. Blum and G. Hogaboom, Principles of Electroplating and Electroformings McGrawHiU Book Co., Inc., New York, 1924. 

A.STM. B503, Std. Practicefor Use of Copper and Hickel Electroplating Solutionsfor Electroforming, American Society for Testing and Materials, Philadelphia, Pa., 1984 Ref 57, Chapts. 13, 14, and 15. 

Sieves are often referred to by their mesh size, which is the number of wires per hnear unit. The U.S. Standard Sieve Series as described by the American Society of Testing and Materials (ASTM) document E-11-87 Standard Specification for Wire-cloth Sieves for Testing Purposes addresses sieve opening sizes from 20 [Lm (635 mesh) to 125 mm (5.00 in). Electroformed sieves with square or round apertures and tolerances of 2 [Lm, are also available. 

The second category was concerned with adhesion to porous or microfibrous surfaces on metals. Aluminium may be anodised to form an oxide surface comprising pores of diameter of tens of nanometers. Electroforming and chemical oxidation can be used to produce microfibrous or needle-like coatings on metals, including copper, steel and titanium. The substrate topography was demonstrated to play an vital part in adhesion to these surfaces [45-48]. 

Nickel is also widely used as an electrodeposited underlay to chromium on chromium-plated articles, reinforcing the protection against corrosion provided by the thin chromium surface layer. Additionally the production of articles of complex shape to close dimensional tolerances in nickel by electroforming —a high-speed electrodeposition process —has attracted considerable interest. Electrodeposition of nickel and the properties of electro-deposited coatings containing nickel are dealt with in greater detail in Section 14.7. 

Discs Discs of nickel electroformed on to mandrels bearing grooves modulated with recorded sound have been used for many years for stamping sound recording discs. This process has been adapted and refined for the manufacture of digital records, including video discs . Video disc stampers must be hard, stress-free, and flat to within 0.1 m results of a short investigation directed towards these requirements have been reported ". 

Other applications of nickel electroforms are reviewed in Reference 75. 

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